1. Field of the Invention
The present invention relates to the technology of measuring the wavelength characteristics of an optical filter and a light transmission channel using an optical spectrum analyzer which measures the spectra of optical signals and a wavelength tunable light source capable of outputting light at different wavelengths.
2. Description of the Related Art
FIG. 4 is a block diagram showing the configuration of a prior art wavelength tracking system using an optical spectrum analyzer and a wavelength tunable light source. Indicated by 1 is an optical spectrum analyzer, 2 is a control unit in the optical spectrum analyzer 1, 3 is a communication circuit for performing communication with external equipment, {circle around (2)} is an I/O terminal providing an interface with the communication circuit 3, 4 is a spectroscope which performs spectral resolving to extract a specified wavelength from the light to be measured and outputs the extracted wavelength, 5 is a light input terminal at which the light to be measured is input to the spectroscope 4, 6 is a motor for tuning the wavelength being extracted with the spectroscope 4, 7 is a drive circuit for driving the motor 6, 8 is a photodetector that receives the extracted light being output from the spectroscope 4 and which converts it to an electrical signal, 9 is an amplifier circuit for amplifying the small electrical signal being output from the photodetector 8, 10 is an A/D converter with which the analog signal being output from the amplifier circuit 9 is converted to a digital signal, 11 is a display unit for displaying the measured data, 12 is a position detector circuit for detecting the rotating position of the motor, 14 is a wavelength tunable light source, 15 is a control unit in the wavelength tunable light source 14, 16 is a communication circuit for performing communication with external equipment, {circle around (2)}xe2x80x2 is an I/O terminal providing an interface with the communication circuit 16, 17 is a display unit for displaying conditions and the like, 18 is a light source drive circuit for driving a light source unit 20 and performing, for example, temperature control of the light source unit, 19 is a wavelength control circuit for controlling the wavelength being output from the light source unit, 20 is the light source unit that oscillates a single-mode spectrum and which is so adapted as to be capable of turning the oscillating wavelength, 21 is a light output terminal at which the optical signal being output from the light source unit 20 is output externally, and 22 is the device under measurement.
On the basis of motor control information preliminarily stored in the control unit 2, the optical spectrum analyzer 1 drives the motor 6 via the drive circuit 7 and sets the wavelength which is to be extracted by the spectroscope 4. The control unit 2 monitors and controls the position information being sent from the position detector circuit 12 coupled to the motor 6 and after confirming the setting of a specified wavelength, reads data from the A/D converter 10, performs arithmetic operations on the data and displays the result in the display unit 11. On the basis of a preset wavelength range and a preset number of measurements, the control unit 2 determines equally spaced wavelengths to be extracted with the spectroscope 4, intermittently determines data for the respective settings of extraction wavelength, and plots the data on the display unit 11 so that it is displayed as a single waveform.
On the basis of the light source drive information and wavelength information that are preliminarily stored in the control unit 15, the wavelength tunable light source 14 controls the light source drive circuit 18 and the wavelength control circuit 19 so as to tune the single-mode oscillating wavelength of the light source unit 20 and the optical power of its oscillation. The control unit 15 performs mathematical operations to determine relevant parameters from the settings of conditions displayed on the display unit 17 and controls the light source drive circuit 18 and the wavelength control circuit 19 so that the light source unit 20 is oscillated at the desired settings of conditions. The control unit 15 is also capable of intermittent tuning over a certain wavelength range at predetermined intervals.
The optical spectrum analyzer 1 functions as a host that controls the wavelength tunable light source 14 connected to external equipment. The control unit 2 in the optical spectrum analyzer 1 sends out a control instruction from the communication circuit 3 and the I/O terminal {circle around (2)} and supplies it to the control unit 15 in the wavelength tunable light source 14 via the I/O terminal {circle around (2)}xe2x80x2 and the communication circuit 16, thereby setting the wavelength and optical power of signal light to be output from the wavelength tunable light source 14.
We now describe the prior art wavelength tracking system using an optical spectrum analyzer and a wavelength tunable light source by referring to FIG. 5, which is a flowchart for the sequence of steps in measurement by the prior art tracking system shown in FIG. 4.
First, the technician (the person who performs measurement) sets a plurality of conditions for measurement (e.g. xcex0 or the wavelength at which the measurement starts, xcexe or the wavelength at which the measurement ends, and the number of samples to be measured) in the spectrum analyzer 1, which then determines wavelength interval xcex94xcex and other parameters by arithmetic operations (this is step S1; in the following description, symbol S is used without being preceded by the word xe2x80x9cstepxe2x80x9d). In accordance with the determined conditions, the control unit 2 sends a signal to the drive circuit 7, drives the motor 6 so that the wavelength to be extracted with the spectroscope 4 is set at the initial value, monitors the position information being output from the position detector circuit 12, and confirms the setting of the initial wavelength. The control unit 2 also sends out a command for shift to the initial wavelength and supplies it to the externally connected wavelength tunable light source 14 via the communication circuit 3 (S2).
By performing arithmetic operations from the command sent from the optical spectrum analyzer 1, the control unit 15 in the wavelength tunable light source 14 determines parameters to be imparted to the light source drive circuit 18 and the wavelength control circuit 19, sets the light source unit 20 at the initial wavelength and, after the end of the setting, sends a SETTING COMPLETE command to the control unit 2 in the optical spectrum analyzer 1 via the communication circuit 16 (S3). In response to a technician""s instruction for starting measurement (S4), the control unit 2 in the optical spectrum analyzer 1 sends a control signal to the drive circuit 7 such that the wavelength xcex being extracted with the spectroscope 4 becomes equal to xcex0, drives the motor 6 while waiting for the end of its necessary motion while monitoring the position information being output from the position detector circuit 12. The control unit 2 also sends out a command for shift to xcex, supplies it to the externally connected wavelength tunable light source 14 via the communication circuit 3 and waits for the return of a WAVELENGTH SETTING COMPLETE command from the wavelength tunable light source 14 (S5).
When wavelength setting has completed with the optical spectrum analyzer 1 and the wavelength tunable light source 14 (S6), the control unit 2 actuates the A/D converter 10 and picks up a digital signal (S7), as well as performs arithmetic operations to determine the value of optical power from factors such as preset conditions for the amplifier circuit 9 and plots the result on the display unit 11 (S8); the control unit 2 then performs an arithmetic operation to determine the next wavelength xcex which is spaced from the wavelength of measurement by xcex94xcex (S9) and performs another setting of the wavelength to be extracted with the spectroscope 4 and the wavelength to be output from the wavelength tunable light source 14. The control unit 2 continues these steps of wavelength setting and data measurement until xcex exceeds xcexe (xcex greater than xcexe) (S10), whereupon the measurement ends (S11).
If desired, the optical spectrum analyzer 1 and the wavelength tunable light source 14 may be controlled by other suitable means such as a separately provided computer.
According to the above-described prior art wavelength tracking system using an optical spectrum analyzer and a wavelength tunable light source, the optical spectrum analyzer I serving as a host controls the action of the wavelength tunable light source 14 via the communication interface and data measurement is performed as the wavelength being extracted with the spectroscope 4 and the output wavelength from the wavelength tunable light source 14 are set intermittently at predetermined intervals within the range of wavelengths to be measured and, what is more, commands are sequentially sent out through the communication interface. However, this is a very time-consuming procedure and much more time is required to measure an increased number of samples.
The present invention has been accomplished under these circumstances and has as an object providing an improved wavelength tracking system using an optical spectrum analyzer and a wavelength tunable light source, in which data being measured is sampled while performing high-speed sweep of a spectroscope in an optical spectrum analyzer and the wavelength of a wavelength tunable light source is swept in synchronism with the sweep of the spectroscope, thereby achieving high-speed wavelength tracking.
Another object of the invention is to provide a system capable of highly accurate wavelength tracking.
The first object of the invention can be attained by the wavelength tracking system which measures light wavelength characteristics using an optical spectrum analyzer that measures the spectra of optical signals and a wavelength tunable light source that can output light at different wavelengths, in which the timing of starting the rotation of a motor for controlling the drive of a spectroscope in said optical spectrum analyzer that extracts a specified wavelength by spectral resolving is allowed to coincide with the timing of starting the sweep of a single-mode oscillating wavelength from a light source unit in said wavelength tunable light source, and control is so performed as to attain wavelength coincidence within the range of measurement during continuous and high-speed sweep.
Very fast wavelength tracking can be accomplished by the system according to the second aspect of the invention and which is configured as shown in FIG. 1. The system comprises a drive circuit 7 which, in order to start sweeping an optical spectrum analyzer 1 and a wavelength tunable light source 14 on the same timing, controls a motor 6 for driving a spectroscope 4 and outputs a certain control signal on the timing where the motor 6 starts to rotate, and a wavelength control circuit 19 which controls the sweep of the wavelength of single-mode oscillation from the light source unit 20 and starts sweeping the light source unit 20 in response to a signal externally supplied to control the timing of sweep start. The system is so adapted that the sweep of the wavelength being measured with the optical spectrum analyzer 1 and that of the wavelength of single-mode oscillation from the wavelength tunable light source 14 are started on the same timing while achieving high-speed sweep.
The system according to the third aspect of the invention is also configured as shown in FIG. 1, except that sweep is started with the drive circuit 7 in the optical spectrum analyzer 1 being synchronized by the control signal being output from the wavelength control circuit 19 in the wavelength tunable light source 14 and control is so performed that the wavelength being extracted with the spectroscope 4 and the output wavelength of the light source unit 20 are brought into coincidence within the range of measurement during continuous and high-speed sweep.
The system according to the fourth aspect of the invention is configured as shown in FIG. 2. During high-speed sweep, the optical spectrum analyzer 1 relies upon a position detector circuit 12 connected to the motor 6 and a switching/delay circuit 28 to obtain the timing of sampling with the A/D converter 10 as well as the timing of allowing the resulting digital signal to be read by the control unit 2. For wavelength tracking of the optical spectrum analyzer 1 and the wavelength tunable light source 14, the system includes the wavelength control circuit 19 that generates a sampling timing correlated to the wavelength of single-mode oscillation from the wavelength tunable light source 14, an output terminal {circle around (2)}xe2x80x2 and an input terminal {circle around (2)} for supplying the optical spectrum analyzer 1 with the control signal from the circuit 19, and a switching/delay circuit 13 which performs switching-based selection between the control signal generated from the position detector circuit 12 and the control signal outputted from the wavelength control circuit 19 and supplied to the optical spectrum analyzer 1 and which imparts a desired amount of delay to the selected control signal before it is supplied to the A/D converter 10. The system is so adapted that the sweep of the wavelength being measured with the optical spectrum analyzer 1 and that of the wavelength of single-mode oscillation from the wavelength tunable light source 14 are started on the same timing to achieve high-speed sweep.
The system according to the fifth aspect of the invention is so configured as shown in FIG. 3. The light source unit in the wavelength tunable light source 14 is composed of a diffraction grating 24 and a laser device 23 and by moving the diffraction grating 24 with a motor 25, the oscillating wavelength of the laser device 23 is tuned. The system further includes a drive circuit 26 that can start the motor 25 in response to a control signal that is output from the drive circuit 7 in the optical spectrum analyzer 1 to indicate the timing of sweep start and a position detector circuit 27 that is connected to the motor 25, detects its rotating position and supplies the detected position information to the switching/delay circuit 28 in the optical spectrum analyzer 1 via the output and input terminals {circle around (2)} and {circle around (2)}xe2x80x2. The system is so adapted that the sweep of the wavelength being measured with the optical spectrum analyzer 1 and that of the wavelength of single-mode oscillation from the wavelength tunable light source 14 are started on the same timing to achieve high-speed sweep.
According to the wavelength tracking systems using an optical spectrum analyzer and a wavelength tunable light source which are configured as shown in FIG. 1, the control unit 2 in the optical spectrum analyzer 1 determines various conditions for measurement by performing arithmetic operations on a preset range of wavelengths to be measured and a preset number of samples, and both the drive circuit 7 for the motor 6 that drives the spectroscope 4 and the position detector circuit 12 that actuates the A/D converter 10 at each wavelength of interest for sampling of data are set in their initial state. The conditions for measurement are transmitted to the control unit 15 in the wavelength tunable light source 14 via the communication circuit 3, whereupon the control unit 15 determines the conditions for controlling the light source unit 20 by performing arithmetic operations on the received conditions for measurement and initializes both the light source drive circuit 18 (for controlling the light source unit 20) and the wavelength control circuit 19, which are then set on standby.
In response to an instruction for starting measurement, the control unit 2 in the optical spectrum analyzer 1 actuates the drive circuit 7 to start the motor 6 so that a control signal for sweep start is output from the drive circuit 7. In synchronism with the control signal being output from the drive circuit 7, the wavelength control circuit 19 in the wavelength control light source 14 starts continuous sweep of the oscillating wavelength of the light source unit 20. The control units 2 and 15 have already performed arithmetic operations to determine those conditions for measurement by which the range of sweep per unit time of the wavelength being extracted with the spectroscope 4 becomes equal to the range of wavelength sweep per unit time of the single-mode signal light oscillated by the light source unit 20. Therefore, if synchronization for sweep start is effected with high precision, the wavelength being extracted with the spectroscope 4 and the oscillating wavelength of the light source unit 20 coincide and wavelength tracking can be performed at high speed without compromising the accuracy of wavelength measurement.
According to the wavelength tracking system using an optical spectrum analyzer and a wavelength tunable light source which is configured as shown in FIG. 2, the A/D converter 10 in the optical spectrum analyzer 1 samples data in response to a control signal that is output from the position detector circuit 12 at each point of data measurement and the control unit 2 picks up the sampled data and displays it on the display unit 11. In order to increase the accuracy of wavelength measurement with reference to the wavelength of the light source unit 20, position information for actuating the A/D converter 10 to effect data sampling is output from the wavelength control circuit 19 in the wavelength tunable light source 14, a control signal is supplied into the A/D converter 10 and the control unit 2 via dedicated input/output terminals and the switching/delay circuit 28, data is sampled in response to the control signal that is output at each point of data measurement, and the sampled data is picked up by the control unit 2.
In the wavelength tracking system using an optical spectrum analyzer and a wavelength tunable light source which is configured as shown in FIG. 3, the light source unit in the wavelength tunable light source 14 consists of the laser device 23 and the diffraction grating 24 which acts as a wavelength filter and the drive circuit 26 that controls the motor 25 for rotating the diffraction grating 24 is supplied with a sweep start signal from the drive circuit 7 in the optical spectrum analyzer 1. In addition, position information for actuating the A/D converter 10 to effect data sampling is output from the position detector circuit 27 connected to the motor 25, a control signal is supplied into the A/D converter 10 and the control unit 2 via dedicated input/output terminals and the switching/delay circuit 28, data is sampled in response to the control signal that is output at each point of data measurement, and the sampled data is picked up by the control unit 2.